1. Field of the Invention
The present invention relates to a circuit breaker and, more specifically, to a circuit breaker having a trip mechanism with a hatchet plate that is acted upon by a spring so that when the circuit breaker is open the spring biases the hatchet plate toward the rest position and when the circuit breaker is closed the spring biases the hatchet plate toward the trip or open position.
2. Background Information
Electrical switching apparatus for opening and closing electric power circuits typically utilize an energy storage device in the form of one or more large springs to close the contacts of the device into the large currents which can be drawn in such circuits. Such electrical switching apparatus includes power circuit breakers and network protectors which provide protection, and electric switches which are used to energize and deenergize parts of the circuit or to transfer between alternative power sources. These devices also include an open spring which rapidly separates the contacts to interrupt current flowing in the power circuit. Either or both of the close spring and open spring can be a single spring or multiple springs and should be considered as either, even though the singular is hereafter used for convenience. The open spring is charged during closing of the contacts by the close spring which, therefore, must store sufficient energy to both overcome the mechanical and magnetic forces for closing as well as charging the open springs. Moreover, the close spring is required to have sufficient energy to close and latch on at least 15 times the rated current.
The operating mechanism for such circuit breakers typically includes a manual handle, and often an electric motor, for charging the close spring. It also includes a latch mechanism for latching the close spring in the charged state, a release mechanism for releasing the stored energy in the close spring, and an arrangement, a pole shaft for example, for coupling the released energy into the moving conductor assembly supporting the moving contacts of the switch. The operating mechanism has four distinct operational phases, or “conditions,” relating to the position of the main contacts, open or closed, and the state of the close spring, discharged or charged. First, there is an open, discharged condition wherein the circuit breaker main contacts are open and the close spring is discharged. To close the main contacts, the close spring is charged resulting in an open, charged condition. After the close spring is actuated, the main contacts are closed and the close spring is discharged resulting in a closed, discharged condition. Finally, the charge spring may be recharged while the main contacts are closed resulting in a closed, charged condition. The operating mechanism does not always pass through each of these conditions in the order set forth above. For example, after the contacts are closed, it is standard practice to charge the close spring again so that the close spring is ready to be used again. If the circuit breaker trips while in the closed, charged condition, the operating mechanism will be moved into the open, charged condition without being in the open, discharged condition.
The operating mechanism includes a latch mechanism. The latch mechanism includes a hatchet plate that is fixed to a hatchet plate pivot pin and structured to move between an open position, a reset position, and a closed position. The status of the hatchet plate is tied to the condition of the operating mechanism, and more specifically to the condition of the main contacts. That is, if the hatchet plate is in the open position, the main contacts will also be in the open condition. When the hatchet plate is in the reset position, the operating mechanism is in the open, charged condition. When the hatchet plate is in the closed position, the main contacts are in the closed condition, although the close spring may be charged or discharged.
The hatchet plate is coupled to the other components of the operating mechanism via a link which, due to its particular shape in the circuit breaker described below, is identified as a “banana link.” The hatchet plate is also coupled to a frame assembly via a spring. In prior art, the rest spring was typically attached to the hatchet plate at the banana link pivot pin and biased the hatchet in the reset direction. The disadvantage to this configuration is that the reset bias of the spring tends to prevent tripping of the circuit breaker under unfavorable conditions of high friction and/or low contact force.
There is, therefore, a need for a spring offset device having an offset member, a spring anchor, and a spring extending therebetween structured so that when the circuit breaker is closed, the spring creates a force on the hatchet plate biasing the hatchet plate toward the open, trip position, but when the circuit breaker is open, the spring creates a force on the hatchet plate biasing the hatchet plate toward the reset position.
There is a further need for a spring offset device that may be easily incorporated into presently existing circuit breakers.